Concentrating fluosilicic acid

ABSTRACT

A process for the concentration of aqueous fluosilicic acid solutions is disclosed. In the process, a dilute aqueous fluosilicic acid solution is premixed with concentrated sulfuric acid under superatmospheric pressure and elevated temperatures; the mixture is introduced to a reduced pressure, high-temperature dehydration zone to release a gaseous overhead containing essentially all of the silicon tetrafluoride and a liquid sulfuric acid bottoms; the gaseous overhead is passed to a scrubbing zone to effect reaction of the silicon tetrafluoride with the water content of a dilute aqueous fluosilicic acid solution and thereby concentrate the fluosilicic acid content therein and the concentrated fluosilicic acid solution is then recovered. Advantageously, the diluted sulfuric acid bottoms can be contacted with a siliceous material to convert any contained hydrogen fluoride to another silicon tetrafluoride-containing gaseous overhead which is also passed to the scrubbing zone to effect further concentration. Hydrogen fluoride may also be recovered in pure form in a particular embodiment.

Parish et al. I

[45'] Feb. 29, 1972 [54] CONCENTRATING FLUOSILICIC ACID [72] Inventors:William R. Parish; James C. Kelley, both of Lakeland, Fla.

[73] Assignee: Wellman-Lord,lnc.

[22] Filed: Mar. 9, 1970 [21] Appl. No.2 17,580

[52] U.S. Cl ..23/153,23/167,23/182, 23/205 [51] lnt. Cl ..C01b 7/22,COlb 33/08, C01b 33/00 [58] Field ol'Search .......23/153,205, 182, 167

[56] References Cited- UNITED STATES PATENTS 3,218,124 l1/1965 Oakley,Jr. et al ..23/153 3,218,126 1l/1965 Wilkinson ..23/153 3,218,12811/1965 Klem ..23/153 1,960,347 5/1934 Osswald et al ..23/l53 2,456,50912/1948 Hopkins, Jr. et a1 ..23/153 3,415,039 12/1968 Rushton et al..23/l53 X 3,233,969 2/1966 Heller et a1. ..23/182 3,218,125 11/1965Houston et a1 ..23/153 Primary Examiner-Edward Stern Attorney-Morton,Bernard, Brown, Roberts & Sutherland, John W. Behringer, Martin .1.Brown, W. Brown Morton, Jr., Eugene L. Bernard, James N. Dresser, JohnT. Roberts and Malcolm L. Sutherland [5 7] ABSTRACT A process for theconcentration of aqueous fluosilicic acid solutions is disclosed. In theprocess, a dilute aqueous fluosilicic acid solution is premixed withconcentrated sulfuric acid under superatmospheric pressure and elevatedtemperatures; the mixture is introduced to a reduced pressure,high-temperature dehydration zone to release a gaseous overheadcontaining essentially all of the silicon tetrafluoride and a liquidsulfuric acid bottoms; the gaseous overhead is passed to a scrubbingzone to effect reaction of the silicon tetrafluon'de with the watercontent of a dilute aqueous fluosilicic acid solution and therebyconcentrate the fluosilicic acid content therein and the concentratedfluosilicic acid solution is then recovered. Advantageously, the dilutedsulfuric acid bottoms can be contacted with a siliceous material toconvert any contained hydrogen fluoride to another silicontetrafluoride-containing gaseous overhead which is also passed to thescrubbing zone to effect further concentration. Hydrogen fluoride mayalso be recovered in pure form in a particular embodiment.

21 Claims, 2 Drawing Figures SCRUBBING Sim/ PRODUCT ZONE ZONE STORAGE F44 I I I 43 47 304 l 26 ABSORP'HON DESORPTION '2 1 7 DEFLUORINATOR ZONEZONE 21 25 3 DEHYDRATION MIXER ZONE PATENTEDFEB29 I972 FIG. I 32 s g as33] SCRUBBING SEPARATOR wlfl, PRODUCT ZONE STORAGE t -l6 l- 61 1 DEH nATION I MIXER 7 YR DEFLUORlNATOR ZONE SCRUBBING V gmg PRODUCT ZONE ZONESTORAGE ABSORPTION DESORPTION 1 7 DEFLUORINATOR ZONE zone EH DRATI N LMIXER D Y 0 ZONE ATTORNEYS INVENTORS WILLIAM R. PARISH JAMES C. KELLEYCONCENTRATING FLUOSILICIC ACID Other applications, commonly ownedherewith, concerning processes of treating fluosilicic acid are U.S.Ser. No. 812,229, filed Apr. 15, 1969; Us. Ser. No. l7,590, filed Mar.9, 1970', and US. Ser. No. 17,61 l,filed Mar. 9, 1970.

This invention relates to a process for the concentration of diluteaqueous solutions of fluosilicic acid by dehydration with concentratedsulfuric acid to prepare concentrated aqueous solutions of fluosilicicacid. Diluted sulfuric acid suitable for phosphoric acid manufacture isalso produced and, in a particular embodiment, hydrogen fluoride mayalso be recovered.

The process of the present invention involves mixing strong sulfuricacid, including oleum, having a concentration generally of at leastabout 85, say about 85 to 100, preferably about 90 to 99, weight percentof sulfuric acid with dilute aqueous solutions of fluosilicic acidhaving a composition ranging normally from to 30 weight percent H SiFand 70 to 90 weight percent water. Dilute aqueous fluosilicic acidsolutions containing from about 10 to 30 weight percent H SH-1 and 70 to90 weight percent water are normally produced as a byproduct inphosphoric acid manufacture. Preferably, the sulfuric acid and diluteaqueous fluosilicic acid are each preheated to about 90 to 240 F., priorto their being admixed together. The weight ratio of the-acids is suchthat the spent acid mixture, after the separation step describedhereinafter, hasa sulfuric acid concentration of at least about 70weight percent and advantageously about 70 to 95 weight percent.Theweight ratio on an anhydrous basis is thus normally at least 5 toabout 30 parts, preferably about to parts, sulfuric acid per part offluosilicic acid.

The mixing of the two acids is carried out under superatmosphericpressure for a time to allow substantially complete mixing of the acids.The temperature of the mixture can range from ambient temperatures up toabout 350 F. At temperatures about about 350 F., the construction ofsuitable apparatus becomes difficult from a materials standpoint.Preferably, the temperature of the mixture will reach the temperature ofthe liquid in the dehydration zone, which is hereinafter discussed. Thetime required for the acid mixture to reach a suitable temperature willoften be about 0.1 to 10 seconds or more. The amount of pressureemployed is at least that which is sufficient to prevent the water inthe acid mixture from reacting with the silicic component to producesilicon dioxide, commonly referred to as silica. For example, at aboutatmospheric pressure and elevated temperatures comparable to thoseherein, SiF (from the fluosilicic acid) and water can react to formsilica. The formation of silica is essentially avoided in the process ofthe present invention by maintaining the pressure above atmospheric,e.g., at least about 5 pounds per square inch gauge (psi) and preferablyat least about 10 or 15 p.s.i.g.

The pressurized mixture of the sulfuric acid and fluosilicic acid isnext introduced into a separation zone maintained at lower pressure thanthe mixing zone, whereby a gaseous overhead is evolved and liquidbottoms are formed. The temperature of the liquid in the separation zoneis normally about ambient to 325 F., and thus is below the boiling pointof the sulfuric acid in the zone. The pressure in the separation zone issufficiently below that of the pressurized mixture zone so as to allowgas-liquid separation. Atmospheric pressure is normally advantageous.The gaseous overhead produced in the separation zone containsessentially all of the silicon tetrafluoride produced from the mixing ofthe sulfuric and fluosilicic acids. The gaseous overhead also cancontain hydrogen fluoride in an amount ranging from a very minor amount,e.g., essentially nil, to essentially all of the hydrogen fluorideproduced from the admixing of the acids. The amount of hydrogen fluoridepresent in the gaseous overhead is predominantly dependent upon thetemperature of the liquid in the separation zone. At temperatures withinthe lower portion of the range described hereinbefore, i.e., up to about250 F., essentially all or at least a substantial portion of thehydrogen fluoride is present in the dilute sulfuric acid bottoms, whileat temperatures within the upper portion of that range, i.e., betweenabout 250 to 350 F., most or significant amounts of the hydrogenfluoride is present in the gaseous overhead. The amount of hydrogenfluoride liberated in the gaseous overhead is not particularlysignificant to the operation of the overall concentration process ofthis invention; however, more specific embodiments described hereinaftermay require minor or major portions of hydrogen fluoride in the gaseousoverhead.

The separation zone is preferably one which allows the mixture to besufficiently dispersed such that it has at least about one, preferablyat least about 20, or even at least about 40. square centimeters ofliquid gas interface per cubic centimeter of liquid. in other words, thevessel which houses the separation zone is one which provides a largesurface area for the liquid, examples of such being spray towers,falling film evaporators, wiped film evaporators, forced circulationevaporators or any highly agitated vessel. In a falling film evaporator,for instance, there can often be obtained about 20 to 40 squarecentimeters of liquid gas interface per cubic centimeter of liquid.

The desired temperature can usually be maintained in the separation zonewithout the application of external heat. As mentioned above, it ispreferred that the entering mixture of acids already be up to thedesired separation zone temperature. The temperature of the zonescontents depends mainly upon the inlet temperatures of both solutions,the heat of dilution of sulfuric acid and the heat of vaporization ofthe SiF, gas. The heat of vaporization of the SiF, gas tends tocounterbalance the heat of dilution of the sulfuric acid. By preheatingeither or both acids and by maintaining them in pressurized admixturewith one another for a sufficiently long time, the objective of bringingthem up to separation zone temperatures is greatly facilitated.

The residence time of the mixture in the separation zone is relativelyshort, ranging, for example, from about 01 minute to about minutes,preferably about 0.5 to 5 minutes. The fluosilicic acid is dehydratedduring the time beginning with its admixture with the concentratedsulfuric acid and extending to its residence in the separation zone, andthe sulfuric acid is correspondingly diluted. Hydrogen fluoride andsilicon tetrafluoride and water are the products of the dehydration offluosilicic acid. As noted before, silicon tetrafluoride exits theseparation zone as substantially anhydrous gaseous overhead. Thehydrogen fluoride remains in solution or can also be removed as gaseousoverhead, depending on the temperature in the separation zone. Thediluted sulfuric acid is withdrawn from the separation zone as liquidbottoms, for example, having a sulfuric acid content of at least about70, often about 70 to 95, percent, based on the combined weight of waterand sulfuric acid. This concentration can be controlled by adjusting theratio of the sulfuric acid to the fluosilicic acid in the pressurizedmixture introduced to the separation zone.

The diluted sulfuric acid bottoms from the separation zone will containfrom a very minor amount of hydrogen fluoride, say about 1 percent ofthe total amount produced during the dehydration, to essentially all,say about 99 percent of the total amount produced during dehydration.Dilute sulfuric acid bottoms which contain hydrogen fluoride are passedto a spent acid hold tank and maintained at a temperature below theboiling point of the sulfuric acid, normally within the range of about100 to 300 F., at essentially atmospheric pressure. In the hold tank,the dilute sulfuric acid bottoms may be treated with a siliceousmaterial such as sand, precipitated silica, or diatomaceous earth,advantageously under turbulent conditions, to convert any hydrogenfluoride present in the bottoms to silicon tetrafluoride, which, due toits low solubility in the dilute sulfuric acid bottoms, is evolvedtherefrom as a gas. The amount of silica added to the spent acid holdtank is preferably at least about stoichiometric to the amount of HF inthe sulfuric acid.

The amount of sulfuric acid in the dilute sulfuric acid bottoms isadvantageously above about 65 percent, based on the combined weight ofwater and sulfuric acid. Above about 65 weight percent sulfuric acid,the subsequent silica treatment gives silicon tetrafluoride as theproduct, as follows:

l. 4HF SiO SiF, 2H O However. below about 65 weight percent sulfuricacid, the reaction proceeds as follows: a

2. 6Hf+ SiO H SiF 2H O Thus, the concentration of the sulfuric acid inthe sulfuric acid bottoms to be silica-treated should advantageously beabove about 65 weight percent, preferably above about 70 weight percent,so that silicon tetrafluoride is produced. The silicon tetrafluoride isremoved as a gaseous overhead; this product is then directly convertedinto fluosilicic acid in the scrubbing or concentration zone describedhereinafter.

The fluorine level present in the dilute sulfuric acid bottoms aftersilica treatment is normally less than about 0.2 percent by weight. Thislow fluorine, dilute sulfuric acid bottoms can be useful in phosphoricacid manufacture and sometimes may also be admixed with concentratedsulfuric acid to form a suitable sulfuric acid feed for the presentconcentration process.

The gaseous overhead from the separation zone, containing essentiallyall of the silicon tetrafluoride from the dehydration of the fluosilicicacid feed, and, optionally, the gaseous overhead from the spent acidhold tank produced by the dehydrofluorination, if any, of the dilutedsulfuric acid bottoms from the separation zone, are contacted with awash liquid of dilute, aqueous fluosilicic acid solution in thescrubbing zone. The fluosilicic acid wash solution may advantageously beof the same composition as previously described for the feed in theprocess of this invention. In the scrubbing zone, the silicontetrafluoride in the gaseous overheads reacts with water to formfluosilicic acid and silica, which may or may not precipitate, dependingupon the hydrogen fluoride content of the gaseous overhead from theseparation zone. The reactions in the scrubbing zone will be describedin greater detail hereinafter.

The scrubbing zone serves to concentrate the fluosilicic acid content ofthe scrubbing solution. Depending upon the concentration of hydrogenfluoride in the gaseous overhead, the concentrating is accomplished bythe occurrence of the following reactions in varying degrees:

3. 3SiF. 2H O ZH SiF SiO,, 4. 6HF SiO H SiF 2H O 5. 4HF sio siF, 2H O 6.2H +siF, H SiF Furthermore, depending upon the hydrogen fluoride contentof the overhead, silica may or may not precipitate in the hydrationzone, as mentioned hereinbefore.

The scrubbing zone is normally maintained within the temperature rangeof about 75 to 150 F., preferably below about 120 F. The scrubbing zonecan be supplied by any suitable gas-liquid contacting vessel as, forexample, one or more spray towers. Additionally, any silicontetrafluoride gas passing through the scrubbing zone unreacted may berouted to a second such scrubbing zone to reduce SiF, losses.

If the hydrogen fluoride content of the overhead from the dehydrationzone is not sufficient to convert essentially all of the silica formedin the scrubbing zone to SiF then the concentrated fluosilicic acidleaving the scrubbing zone may be treated to remove the precipitatedsilica content. The separation of the silica from the fluosilicic acidproduct is accomplished in a silica separation zone by, for example,centrifugation, filtration or decantation. The separated silica may thenoptionally be washed to remove absorbed, concentrated fluosilicic acid.The washing is normally accomplished by treatment with weak fluosilicicacid; preferably, the weak fluosilicic acid solution entering theinitial scrubbing zone is used. Also, the separated silica, washed orunwashed, may be then used in the silica treatment of the dilutedsulfuric acid bottoms in the hold tank to transform hydrogen fluoride tosilicon tetrafluoride.

The fluosilicic acid solution removed from the scrubbing zone, or, ifutilized, from the silica separation zone, is the concentratedfluosilicic acid product of this invention. The concentrated productusually is at a temperature of about 75 to 125 F., and normally about 90to 100 F. This product is then passed to a suitable storage zone. Theconcentrated fluosilicic acid solution prepared by the process of thisinvention has a fluosilicic acid content ranging from about 20 to morethan about 70 percent by weight, depending upon the particular feed,scrubbing solution used, and other process variables. The fluosilicicacid concentration of the concentrated solution product is always higherthan the fluosilicic acid concentration of the dilute solution feed.Normally. the product will have a fluosilicic acid content of from about40 to 60 percent by weight.

In an alternative embodiment, hydrogen fluoride. as well as theconcentrated fluosilicic acid, can be recovered as a product of theprocess of this invention. In this embodiment, the gaseous overhead fromthe dehydration zone contains recoverable amounts of hydrogen fluorideas well as silicon tetrafluoride, and this overhead is treated in an HFabsorption zone to remove hydrogen fluoride prior to passage of thesilicon tetrafluoride to the scrubbing zone. In the absorption zone, thegaseous overhead from the separation zone is contacted with liquidsulfuric acid having a concentration of about 93 to 99, preferably about98 to 99, weight percent and at temperatures up to about 120 F., oftenabout 90 to 100 F., to selectively absorb the HF. Any suitablegas-liquid contacting vessel can be employed. The vessel is preferablyone in which the gaseous overhead is passed upwardly through adescending stream of the concentrated sulfuric acid, e.g., in a packedtower, sieve tray column, bubble-capped column, spray tower, etc.Silicon tetrafluoride passes through the sulfuric acid without beingabsorbed. Preferably, there is employed sufficient sulfuric acid in thisabsorption step to absorb at least about 80, most preferably at leastabout 90, percent of the HF in the overhead stream from the separationzone.

Much less sulfuric acid is required to accomplish this than is needed todehydrate the fluosilicic acid as described above. Thus, for example, ofthe total amount of concentrated sulfuric acid used in the two steps,usually only about 1 to l0, e.g., about 5 percent, will be used in theHF absorption step. The spent sulfuric acid obtained from the absorptionstep will often contain about 2 to 40, usually about 10 to 20, weightpercent of dissolved HF.

The HF-containing sulfuric acid from the above absorption step is thenconducted to a desorption zone wherein it is heated to a temperaturesufficient to liberate gaseous hydrogen fluoride therefrom, but belowthe boiling point of the solution in the zone; often, for example, atemperature of about 200 to 300 F. will be suitable, especially atapproximately atmospheric pressure. The HF which'is recovered therebycan be about 98 to 99 percent pure. To purify it further, say to about99.8 percent purity, it can be rectified. An advantage of rectifying theHF at this point in the process rather than condensing the HF whileadmixed with the SiF, in the overhead from the separation zone, is that98 percent pure HF condenses about 68 F. at atmospheric pressure,whereas the I-lF/SiF overhead mixture would have to be cooled to aboutminus 55 F. in order to condense the HF therefrom.

The hydrogen-fluoride-desorbed sulfuric acid can be passed to the spentacid holding tank where it is mixed with the dilute sulfuric acidbottoms from the dehydration zone. The resultant acid mixture can betreated with a siliceous material, as described before, for theproduction of SiF, from any HF present.

The process of the instant invention can be more readily described byreference to the drawings in which FIG. 1 sets forth a flowsheetillustrating one embodiment of the invention, and

FIG. 2 sets forth an alternative embodiment of the invention.

Referring to FIG. 1, fluosilicic acid, at a temperature of about 90 to240 F. and a concentration of from about 10 to 30 weight percent H SiFis carried in line 14 to mixing tee 27 where it is combined with to 100percent sulfuric acid introduced via line 2, heater 28 and line 24, alsoat a temperature of from about to 240 F. The weight ratio (anhydrous) ofsulfuric acid to fluosilicic acid in the mixture is from about 5:1 to30:1. The mixture of acids is conducted via line 23 under about 5 to l5p.s.i.g. pressure to the dehydrator 25, which can be a falling film typeof evaporator. Residence time in line 23 is about 5 seconds and themixture enters the evaporator at about 200 to 300 F., which isapproximately the temperature of the liquid in the evaporator. SiF, gasflashes off under the approximately atmospheric pressure conditionsmaintained in the evaporator and is removed via line 15. Dilutedsulfuric acid (about 80 to 85 weight percent concentration) is withdrawnas bottoms via line 3. Residence time of the reactants in the evaporatoris about 2 to 5 minutes.

The dilute sulfuric acid bottoms in line 3 are passed to the spent acidhold tank 26 in which the acid is treated with a siliceous material,such as sand, introduced through line 29. The silica-treated acidmixture is maintained under turbulent conditions, as by a stirrer (notshown), at a temperature of from about 200 to 300 F. and at essentiallyatmospheric pressure. The silica reacts with any hydrogen fluoridepresent to produce silicon tetrafluoride which is removed as gaseousoverhead through line 6. The amount of silica introduced is at leastabout stoichiometric to the amount of HF in the dilute sulfuric acidbottoms. The silica-treated dilute sulfuric acid of a concentration ofabout 80 to 85 weight percent can be removed through line 7 for ultimaterecovery. I

The gaseous overheads from the separation zone 25 in line and spentacidhold tank 26 in line 6 are passed into scrubbing zone 32 where theyare contacted with dilute, aqueous fluosilicic acid wash liquid suppliedthrough line 33. The wash liquid is preferably of the same concentrationas the dilute fluosilicic acid solution feedstock l4 and can be takenoff line 14 through line 16. The gaseous overhead from scrubbing zone32, which may contain some unreacted SiF can be passed into a secondsuch scrubbing zone (not shown) to reduce Sil losses.

The concentrated fluosilicic acid solution, of a concentration of fromabout to 70, preferably 40 to 50, weight percent fluosilicic acid, isremoved from scrubbing zone 32 I through line 10 to concentrated acidstorage tank 38.

In an optional embodiment, shown by broken lines in the drawing, inwhich the HF content of the overhead from the dehydration zone and thescrubbing liquid is not sufficient to consume the silica formed in thescrubbing zone 32, the concentrated fluosilicic acid product leaving thescrubbing zone 32 through line 10 is conveyed to a silica separationzone 39 where the silica is removed from the concentrated fluosilicicacid. The silica separation zone 39 can contain suitable filtering,centrifuging or decantation apparatus. The separated silica can bepassed through line 40 to spent acid holding tank 26 as part of thesilica charge to the dilute sulfuric acid bottoms. The separated silicacan also, if desired, be washed with a suitable wash liquid, such as thedilute fluosilicic acid feed solution, to remove absorbed, concentratedfluosilicic acid product solution before being introduced into the spentacid holding tank 26. The silica-removed, concentrated fluosilicic acidproduct can be conveyed to storage tank 38 through line 41.

HO. 2 illustrates an alternative embodiment of this invention in whichHF is also recovered as a product. Fluosilicic acid, at a temperature ofabout 90 to 240 F. and a concentration of from about 10 to weightpercent H SiF is carried in line 14' to mixing tee 27' where it iscombined with 85 to 100 percent strength sulfuric acid introduced vialine 2, heater 28' and line 24', also at a temperature of from about 90to 240 F. The weight ratio (anhydrous) of sulfuric acid to fluosilicicacid in the mixture is from about 5:1 to 30:]. The mixture of acids isconducted via line 23 under about 5 to 15 p.s.i.g. pressure to thedehydrator 25' which can be a falling film type of evaporator. Residencetime in line 23' is about 5 seconds and the mixture enters theevaporator at about 250 to 300 R, which is approximately the temperatureof the liquid in the evaporator. SiF. and HF gases flash off at thesetemperatures and under the approximately atmospheric pressure conditionsmaintained in the evaporator and are removed via line 42. Dilutesulfuric acid (about to weight percent concentration) is withdrawn asbottoms via line 3'. Residence time of the reactants in the evaporatoris about 2 to 5 minutes.

The SiF, and HF gases in line 42 are introduced into absorption zone'43'where they are contacted with liquid sulfuric acid of a concentration ofabout 93 to 99 percent H 50 introduced via line 50 at a temperature ofup to about F. The absorption zone 43' can be any suitable gas-liquidcontact apparatus and preferably is a spray tower, buttle-capped column,sieve-tray tower, packed tower, etc., in which the mixed gases passupwardly through a descending stream of the concentrated sulfuric acid.The HF component of the mixed gases is absorbed in the concentratedsulfuric acid. The HF- containing sulfuric acid is passed via line 46'to desorption zone 47 where the HF is separated from the concentrated H80 In the desorption zone 47', the HF-containing sulfuric acid can beheated to a temperature sufficient to liberate the HF but below theboiling point of the solution. At atmospheric pressure, a temperature offrom about 200 to 300 F. is suitable. The HF recovered from desorptionzone 47 is usually about 98 to 99 percent pure and can be conducted vialine 49' to a HF recovery zone (not shown). The HF-depleted H 80 can beconveyed from desorption zone 47 via line 48 to spent acid hold tank 26where it is mixed with the dilute sulfuric acid bottoms introduced vialine 3 from the dehydration zone 25. The sulfuric acid in tank 26' canbe treated with silica in the same manner as in the embodiment ofFIG. 1. Silica is introduced via line 29 and also can be introduced vialine 40' from silica separation zone 39. The dilute-silica-treatedsulfuric acid of a concentration of about 70 to 75 percent can beremoved through line 31 for ultimate recovery. The SiF produced in tank26 is removed through line 30, mixed with the SiF, removed from theabsorber 43' via line 44 and conveyed via line 45' to scrubbing zone 32for the concentration of fluosilicic acid in the same manner as theembodiment of FIG. 1.

The fluosilicic acid wash liquid to scrubbing zone 32' is preferably ofthe same composition as the dilute fluosilicic acid feedstock 14 and canbe taken off line 14 through line 16'. The concentrated fluosilicic acidsolution product can be conveyed via line 37 to silica separation zone39 for separation ofsilica in the same manner as in the embodiment ofFIG. 1. The silica-depleted concentrated fluosilicic acid product isthen conveyed via line 41' to product storage tank 38.

The concentration process of this invention is advantageous forintegrated use in the preparation of superphosphate fertilizers. Forinstance, the diluted fluosilicic acid solutions useful herein as feedsmay be the byproducts of the preparation of such fertilizers, while thesulfuric acid bottoms, after removal of hydrogen fluoride, are of suchconcentration that they are directly usable in the preparation ofphosphoric acid, of which substantial portions of the total yearlyproduction are used in the preparation of superphosphate fertilizers.

Also, the fluosilicic acid concentrate of this invention is of suchconcentrations that considerable savings in transportation costs areachieved. For instance, by increasing the concentration of fluosilicicacid from 25 to 50 percent by weight in commercial grades of fluosilicicacid solutions, substantial reductions in the water content areachieved. As a result of these reductions, the transportation cost perpound of fluorine can be reduced by about 50 percent. This reduction intransportation costs per pound of fluorine is significant in light ofthe large amounts of fluosilicic acid solutions used today in thefluoridation of water supplies.

The concentrated fluosilicic acid solutions produced by the process ofthis invention are also advantageous for direct use in the preparationof anhydrous hydrogen fluoride and can be used, for example, as thefeedstock in the HF-producing process disclosed in copendingapplication, Ser. No. 816,229, filed Apr. 15, 1969, in the names ofWilliam R. Parish, James C. Kelley, Albert Giovanetti and William A.Lutz.

The following. example serves to describe preferred procedures foraccomplishing the concentration process of this invention.

EXAMPLE I The process flow for this example is illustrated by FIG. 1. Bythis embodiment of the invention, substantially pure (i.e., theprecipitated silica has been removed), concentrated fluosilicic acid isprepared. The process conditions are as follows:

Weight Percent I. Feed:

25% H,SiF,, SiF, 8.70 HF 18.31 H O 72.99 98% H,SO, H 50 97.9

H,O 2.l 2. Fluosilicic Acid Temperature 90 F.

prior to mixing, tc 3. Sulfuric Acid temperature 90 F. prior to mixing,4. Separation zone temperature 220 F.

5. H,SO concentration of spent 80% acid from the separation zone.

6. H,SO.concentration of spent 70% acid from silica treatment 7.Scrubbing liquid temperature ll20 F.

in the scrubbing zone 8. Concentration of H SiF in 40 -50% product 9.Product temperature 90l00 F.

EXAMPLE I] The process flow for this example is illustrated by FIG. II.By this embodiment of the invention, both concentrated fluosilicic acidand hydrogen fluoride are products. The process conditions are asfollows:

Weight Percent Product temperature EXAMPLE Ill The process flow for thisexample is illustrated by FIG. 1. By this embodiment of the invention,the separated silica is washed with weak fluosilicic acid which is thenconveyed to the initial scrubbing zone. The process conditions aresubstantially identical to those in Example 1.

EXAMPLE IV The process flow for this example is also illustrated byFIG. 1. In this embodiment of the invention, the silica separation zoneis eliminated by allowing sufficient hydrogen fluoride content to bepresent in the gaseous overhead from the separation zone to react withand consume the silica produced in the scrubbing zone. The processconditions are as follows:

Weight Percent I. Feed:

25% H,SiF SiF 8.70 HF [8.31 H 0 72.99 98% H 50, H,SO 97.9

2. Fluosilicic acid temperature 6. H,SO concentration from the hold tankafter silica treatment 7. Scrubbing liquid temperature in scrubbing zonel0O-l20 F 8. Concentration of H SiF in product 40-50% 9. Producttemperature 90-|00 F.

It is claimed:

1. A process for concentrating a dilute, aqueous fluosilicic acidsolution which contains about 10 to 30 weight percent H SiF whichprocess comprises:

i. mixing sulfuric acid having a concentration of above about weightpercent with a first portion of said dilute fluosilicic acid solution toeffect dehydration of the fluosilicic acid and dilution of the sulfuricacid. said mix ing being effected under superatmospheric pressure for atime sufflcient to allow substantially complete mixing of the acids,said pressure being sufficient to prevent the water in the mixture fromevaporating to react with the silicic component evolving from themixture which can result in the production of a substantial amount ofsilica;

ii. introducing the mixture into a reduced pressure separation zonewhereby silicon tetrafluoride is evolved and removed from the zone as agaseous overhead and dilute sulfuric acid is withdrawn from the zone atbottoms;

iii. scrubbing said silicon tetrafluoride gaseous overhead of (ii) witha second portion of said dilute, aqueous fluosilicic acid solution in ascrubbing zone to effect reaction between said silicon tetrafluoride andthe water of the fluosilicic acid scrubbing solution to form, as a spentscrubbing solution, a more concentrated, aqueous fluosilicic acidsolution; and

iv. recovering the concentrated, aqueous fluosilicic acid solution fromthe scrubbing zone.

2. The process of claim 1 wherein the temperature of the separation zoneis about 200 to 250 F., the hydrogen fluoride formed by the dehydrationof fluosilicic acid remains in solution with the dilute sulfuric acidand the hydrogen-fluoridecontaining, dilute sulfuric acid bottoms aretreated in a dehydrofluorination zone with a siliceous material torelease the hydrogen fluoride therein as asilicon-tetrafluoride-containing gaseous overhead, while maintaining thedehydrofluorination zone temperature at less than the boiling point ofthe sulfuric acid solution therein.

3. The process of claim 2 wherein the silicon-tetrafluoridecontaininggaseous overhead from the dehydrofluorination zone is conducted to thescrubbing zone to effect reaction between the said silicon tetrafluorideand the water of the fluosilicic acid scrubbing solution to formconcentrated, aqueous fluosilicic acid solution.

4. The process of claim 1 wherein the sulfuric acid is mixed with thefluosilicic acid in step (i) in an anhydrous weight ratio of sulfuricacid to fluosilicic acid of about 5 to 30:1, sufficient to provide thedilute sulfuric acid bottoms with at least about 65 percent sulfuricacid, based on the combined weight of sulfuric acid and water therein.

5. The process of claim 4 wherein in step (i), prior to the mixing, thesulfuric acid and fluosilicic acid are at a temperature of about to 240F.

6. The process of claim 1 wherein the temperature of the separation zoneis about 250 to 300 F., the hydrogen fluoride formed by the dehydrationof fluosilicic acid is also evolved as a gaseous overhead and the mixedgaseous overhead, containing silicon tetrafluoride and hydrogen fluoridefrom the separation zone, is contacted with liquid sulfuric acid havinga concentration of about 93 to 99 weight percent sulfuric acid at atemperature of up to about F. in a hydrogen fluoride absorption zone toselectively absorb the hydrogen fluoride in the sulfuric acid prior toconducting said silicon-tetrafluoridecontaining gaseous overhead to thescrubbing zone of step (iii).

7. The process of claim 6 wherein the hydrogen fluoride in the spentsulfuric acid from the hydrogen fluoride absorption zone is liberatedfrom the spent acid and recovered in substantially pure form by heatingthe spent acid in a desorption zone to a temperature below the boilingpoint of the spent acid.

8. The process of claim 1 wherein any precipitated silica formed in thescrubbing zone is separated from the concentrated fluosilicic acidproduct solution in a silica separation zone.

9. The process of claim 2 wherein any precipitated silica formed in thescrubbing zone is separated from the concentrated fluosilicic acidproduct solution in a silica separation zone.

10. The process of claim 9 wherein the separated silica from the silicaseparation zone is at least part of the siliceous material added to thehydrogen-fluoride-containing, dilute sulfuric acid bottoms to releasethe hydrogen fluoride as silicon tetrafluoride.

11. The process of claim 10 wherein the separated silica is washed withdilute, aqueous fluosilicic acid solution having a concentration ofabout 10 to 30 weight percent l-l SiF before addition to thehudrogen-fluoride-containing, fluoride-containing, dilute sulfuric acidbottoms.

12. The process of claim 1 wherein sufficient hydrogen fluoride ispresent in the scrubbing zone to provide a substantially silica-free,concentrated, aqueous fluosilicic acid product solution.

13. The process of claim 1 wherein the pressure in step (i) is at leastabout pounds per square inch gauge.

14. The process of claim 1 wherein the sulfuric acid employed in step(i) has a concentration of above about 90 weight percent sulfuric acid.

15. The process of claim I wherein, in step (i), the mixing undersuperatmospheric pressure is conducted for a time sufficient to allowthe temperature of the mixture to reach at least about the temperatureof the liquid in the reduced pressure separation zone.

16. The process of claim 1 wherein the sulfuric acid is mixed with thefluosilicic acid of step (i) in an anhydrous weight ratio of about 5 to30:1, sufficient to provide the dilute sulfuric acid in step (ii) withabout 70 percent sulfuric acid, based on the combined weight of sulfuricacid and water.

17. The process of claim 1 wherein the pressure maintained in step (ii)is at least about 10 pounds per square inch gauge.

18. The process of claim 1 wherein, in step (ii), the mixing undersuperatmospheric pressure is conducted for about 0.1 to 10 seconds.

19. The process of claim 1 wherein the mixture in the separation zone instep (ii) is sufficiently dispersed that it has at least about 1 squarecentimeter of liquid-gas interface per cubic centimeter of liquid.

20. The process of claim 19 wherein, in step (ii), the mixture has aresidence time in the separation zone of about 0.1 to minutes.

21. The process of claim 1 wherein, in step (ii), the mixture has aresidence time in the separation zone of about 0.5 to 5 minutes and issufficiently dispersed in the zone such that it has at least about 20squarecentimeters of liquid-gas interface per cubic centimeter ofliquid.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,6u-5,678 Dat d February 29, 19 72 Inventor(s) William R. Parish andJames C. Kelley It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 3, "U.S. Ser. No. 812,229" should read 7 --U.S. Ser. No.816,229--.

Column 1, line 35, "about about 350F." should read --above about350'F.-.

Column 6, line 11, "buttle-capped" should read -bubblecapped--. I

Column 7, line +1, "prior Fluosilicic acid temperature" should read-Fluosilicic acid temp eratur e--.

Column 8, line 6, "80-85% the" should .read "80-85%".

Signed and sealed this Lrth day of July 1972.

(SEAL) Attest:

BO BERT GOTT SC HALK EDWARD M.FLHI'Cl-IER, JR.

Commissioner of Patents Attesting Officer ORM 90-1050 (1 USCOMM-DC60376-P69 U 5. GOVERNMENT PR NTING OFF CE: I969 0-366-334

2. The process of claim 1 wherein the temperature of the separation zoneis about 200* to 250* F., the hydrogen fluoride formed by thedehydration of fluosilicic acid remains in solution with the dilutesulfuric acid and the hydrogen-fluoride-containing, dilute sulfuric acidbottoms are treated in a dehydrofluorination zone with a siliceousmaterial to release the hydrogen fluoride therein as asilicon-tetrafluoride-containing gaseous overhead, while maintaining thedehydrofluorination zone temperature at less than the boiling point ofthe sulfuric acid solution therein.
 3. The process of claim 2 whereinthe silicon-tetrafluoride-containing gaseous overhead from thedehydrofluorination zone is conducted to the scrubbing zone to effectreaction between the said silicon tetrafluoride and the water of thefluosilicic acid scrubbing solution to form concentrated, aqueousfluosilicic acid solution.
 4. The process of claim 1 wherein thesulfuric acid is mixed with the fluosilicic acid in step (i) in ananhydrous weight ratio of sulfuric acid to fluosilicic acid of about 5to 30:1, sufficient to provide the dilute sulfuric acid bottoms with atleast about 65 percent sulfuric acid, based on the combined weight ofsulfuric acid and water therein.
 5. The process of claim 4 wherein instep (i), prior to the mixing, the sulfuric acid and fluosilicic acidare at a temperature of about 90* to 240* F.
 6. The process of claim 1wherein the temperature of the separation zone is about 250* to 300* F.,the hydrogen fluoride formed by the dehydration of fluosilicic acid isalso evolved as a gaseous overhead and the mixed gaseous overhead,containing silicon tetrafluoride and hydrogen fluoride from theseparation zone, is contacted with liquid sulfuric acid having aconcentration of about 93 to 99 weight percent sulfuric acid at atemperature of up to about 120* F. in a hydrogen fluoride absorptionzone to selectively absorb the hydrogen fluoride in the sulfuric acidprior to conducting said silicon-tetrafluoride-containing gaseousoverhead to the scrubbing zone of step (iii).
 7. The process of claim 6wherein the hydrogen fluoride in the spent sulfuric acid from thehydrogen fluoride absorption zone is liberated from the spent acid andrecovered in substantially pure form by heating the spent acid in adesorption zone to a temperature below the boiling point of the spentacid.
 8. The process of claim 1 wherein any precipitated silica formedin the scrubbing zone is separated from the concentrated fluosilicicacid product solution in a silica separation zone.
 9. The process ofclaim 2 wherein any precipitated silica formed in the scrubbing zone isseparated from the concentrated fluosilicic acid product solution in asilica separation zone.
 10. The process of claim 9 wherein the separatedsilica from the silica separation zone is at least part of the siliceousmaterial added to the hydrogen-fluoride-containing, dilute sulfuric acidbottoms to release the hydrogen fluoride as silicon tetrafluoride. 11.The process of claim 10 wherein the separated silica is washed withdilute, aqueous fluosilicic acid solution having a concentration ofabout 10 to 30 weight percent H2SiF6 before addition to thehudrogen-fluoride-containing, fluoride-containing, dilute sulfuric acidbottoms.
 12. The process of claim 1 wherein sufficient hydrogen fluorideis present in the scrubbing zone to provide a substantially silica-free,concentrated, aqueous fluosilicic acid product solution.
 13. The processof claim 1 wherein the pressure in step (i) is at least about 5 poundsper square inch gauge.
 14. The process of claim 1 wherein the sulfuricacid employed in step (i) has a concentration of above about 90 weightpercent sulfuric acid.
 15. The process of claim 1 wherein, in step (i),the mixing under superatmospheric pressure is conducted for a timesufficient to allow the temperature of the mixture to reach at leastabout the temperature of the liquid in the reduced pressure separationzone.
 16. The process of claim 1 wherein the sulfuric acid is mixed withthe fluosilicic acid of step (i) in an anhydrous weight ratio of about 5to 30:1, SUFFICIENT to provide the dilute sulfuric acid in step (ii)with about 70 percent sulfuric acid, based on the combined weight ofsulfuric acid and water.
 17. The process of claim 1 wherein the pressuremaintained in step (ii) is at least about 10 pounds per square inchgauge.
 18. The process of claim 1 wherein, in step (ii), the mixingunder superatmospheric pressure is conducted for about 0.1 to 10seconds.
 19. The process of claim 1 wherein the mixture in theseparation zone in step (ii) is sufficiently dispersed that it has atleast about 1 square centimeter of liquid-gas interface per cubiccentimeter of liquid.
 20. The process of claim 19 wherein, in step (ii),the mixture has a residence time in the separation zone of about 0.1 to80 minutes.
 21. The process of claim 1 wherein, in step (ii), themixture has a residence time in the separation zone of about 0.5 to 5minutes and is sufficiently dispersed in the zone such that it has atleast about 20 square centimeters of liquid-gas interface per cubiccentimeter of liquid.